When exactly did that happen? The first iPhone received 3 major updates, more than most Android phones even today.
I doubt very much that Apple is being arbitrary in their support cutoff for phones. If it's capable of the features they build into the OS, it gets support. If I had to make a guess, the next cutoff point (iOS 15?) is going to be a chip having the Bionic Neural Engine, which would make the iPhone 8 the oldest supported phone.
Hopefully we'll see Mac's with longer lifecycles and macOS support as well. I think the Mac side is a tad short at the moment.
Windows has a different reason why they're so focused on desktop BC: lock-in. Had Windows not taken care of BC over decades and a provable track record of that from the beginning of the DOS era, enterprises with tens of thousands of licenses would have fled the ship (and funded WINE with ludicrous amounts of money, probably). On mobile they didn't give a flying finger about BC, and the contrast was noticeable.
Yep, a fatal mistake, made by leadership brought in from outside Windows. Pretty much wrote the epitaph for Windows with that move.
signed: bitter Windows Phone user.
Apple tends to drop support when some hardware feature or limitation doesn't play well with the new OS. For instance, the iPhone 6 only has 1GB of RAM.
Nokia phones are in my opinion way too underrepresented on the market for the support they give.
From a software and developer side, it makes sense to drop 32 bit support early. It's less radically different architectures to target. It means they can guarantee the performance of the os and apps. But as processors got good enough to handle most things, the need to drop the slow ones, and the slow ones holding back the software evolution is minimized.